While you would assume this to be the case, I would have to say that the V6, with a light load, properly maintained, and carefully driven will probably get better mileage than a V8.

The problem is the V6 runs at a higher RPM to do the same job as the V8, given the same axle ratio and transmission. So if you over work the V6, it will actually get poorer mileage, and be more prone to breakdown than the brawnier V8.

So if you think you're going to use the V6 in a heavy work environment and save gas mileage, you may get your feelings hurt. IF you only intend to use it primarily as transportation and light loads, there's a potential to drive wisely and save gas money.

actually, the 4.3 gets almost the same
as you probably know, the 4.3 is a 5.7 without 2 cylinders
in my moms 2000 4 dr 4x4 blazer 4.3 she gets 18 mpg tops on the highway if shes lucky
my dad has a 2004 silverado, excab, 2wd 5.3 he gets about the exact same
i would get the 5.3 in any truck, they are torquey engines,
if i wanted to get a v6 i would buy a suv lol

Speakin from experience, I hate the 4.3 v6. It's good for nothing except saving some money before driving from the dealership. The V8's usually get pretty much the same mileage. I've even noticed a V8 get better mileage when you're talking about pickups. Put 800 lbs in the back and the V8 will do better mileage wise and overall drive satisfaction wise.

my Z71 with HO 5.3 i can average 20-24 mpg on the highway @55 mph depending on where im at with a programmer. 70mph itll get 18. if i lighten my foot and just drive it i normally get around 14-15 going to work and back all week. when my foot gets heavy that can go down to 12.x
gets about 15 towing the race car trailer on the highway.
3.42 gears seems to help alot with the highway mileage but are pretty doggy for towing.